1. Field of the Invention
The present invention relates to an optical scanning device for scanning a member with a light beam to guide the light beam to a predetermined position of the member using a plastic optical element. In addition, the present invention also relates to an optical scanning method. Further, the present invention relates to an image forming apparatus using the optical scanning device.
2. Discussion of the Background
Optical image writing units used for conventional laser digital copiers, printers and facsimiles typically include an optical element having a rectangular form, which focuses a laser beam and which has varieties of correction functions.
Recently, due to needs for cost reduction, the material of such optical elements has been changed from glass to plastics. In addition, the optical elements have varieties of surfaces of from spherical surfaces to complex non-spherical surfaces so that the desired plural functions can be performed with a minimum number of optical elements. For example, varieties of lenses such as lens with large thickness and/or uneven thickness in the longitudinal direction thereof have been used as the optical elements. Such a plastic optical element can be mass-produced at a low cost by inserting a resin in a cavity of a die having the form of the plastic element or injecting a melted resin to the cavity.
In such conventional plastic molding methods, it is preferable that the pressure applied to the resin in the cavity of the die and the temperature of the resin therein have to be controlled so as to be uniform in the cooling/solidifying process of the melted resin in the cavity to prepare a plastic element having a desired form and a high-dimensional precision. However, when a lens with an uneven thickness form is formed by this method, a sink problem in that the portions of the lens having different thicknesses have different amounts of volume contraction, resulting in formation of a lens with a poor dimensional precision and formation of sink on a thicker portion of the lens is caused.
When a technique such that in an injection molding method the injection pressure is increased to increase the amount of the resin injected into the cavity of the die is used to solve the problem, the internal strain of the molded plastic element seriously increases particularly in a case where the plastic element has large thickness or uneven thickness, resulting in deterioration of the optical properties of the plastic element.
Thus, when the injection pressure is decreased to decrease the amount of the resin injected into the cavity, the sink problem is caused. In contrast, when the injection pressure is increased to increase the amount of the resin injected into the cavity, the internal strain of the molded plastic element increases.
A published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2000-84945 corresponding to U.S. Pat. No. 6,919,120 discloses a technique in that a recessed portion or a projected portion is formed on a surface other than the transfer surfaces of the resin element to reduce the internal pressure and the internal strain of the resin. In this regard, the recessed portion is sometimes referred to as a non-transfer surface. By using this technique, plastic elements with a high mechanical precision can be produced at low costs even when the plastic elements have large thickness and/or uneven thickness.
Specific examples of the method for use informing a recessed portion (i.e., non-transfer surface) on a surface other than transfer surfaces include the methods disclosed in JP-As 2000-141413 and 11-28745 (corresponding to U.S. Pat. No. 6,015,514). Specifically, a die in which a cavity is formed by one or more transfer surfaces of a fixed cavity piece and another surface including a non-transfer surface (for forming a recessed portion) formed by a slidable cavity piece is used. A melted resin heated to a temperature higher than the softening point thereof is injected into the cavity while the temperature of the die is controlled to be lower than the softening point of the resin. The injected resin is contacted with the transfer surfaces of the die due to pressure applied to the resin, resulting in transfer of the transfer surfaces to the surfaces of the injected resin. After the resin is cooled to a temperature lower than the softening point thereof, the slidable cavity piece is slid so as to be separated from the resin (i.e., from the cavity) to forcibly form an air gap between a surface of the resin and the slidable cavity piece, i.e., to form a recessed portion (non-transfer surface) on a surface of the resin.
When an plastic element is molded by a general molding method, internal strains and deviations (birefringence) thereof are caused in the longer side direction and in the shorter side direction of the molded element, and thereby the optical properties of the plastic element deteriorate (for example, the beam spot diameter widens). In this regard, the internal strain includes:    (1) An internal strain caused due to orientation of the injected resin in the flowing direction of the resin; and    (2) Another internal strain caused by the stress in the die used.
It is well known that the optical properties of the element are greatly affected by the latter internal strain.
By using the molding methods proposed by JP-As 2000-141413 and 11-28745, the absolute amount of the internal strain in the molded element decreases, but a problem in that the deviation (birefringence) in the shorter side direction of the molded element is greater than that in general molding methods occurs. This is because stress is relieved at the non-transfer surface side of the die (i.e., internal strain at the non-transfer surface side is lower than that in the peripheral portions of the non-transfer surface side), thereby increasing the deviation of the internal strain (i.e., birefringence) in the shorter side direction of the molded optical element. This optical property deterioration problem (particularly, the beam spot widening problem) is prominently caused in high resolution image forming apparatus.
Because of these reasons, a need exists for a molded plastic optical element having good optical properties or an optical scanning device which can maximize the utilization of a molded plastic optical element.